This invention relates to an electromechanical force motor. More particularly this invention relates to an electromechanical driver for hydraulic servoactuators or electrohydraulic flow control valves and the like.
In the past, linear motion electromagnetic devices have been known wherein an armature is surrounded at one end by an annular electromagnetic coil which in turn is encompassed by a concentric permanent magnet having axially directed poles. A pair of Belleville spring washers bias the armature in a first direction and overcome an attractive force exerted upon the armature by the permanent magnet. Upon application of current to the electromagnetic coil, however, the flux of the permanent magnet and electromagnet become additive and serve to overcome the spring bias and produce work through translation of the armature. Depending upon the springs selected, current applied or design of the magnetic members, such a device may be functionally designed for an off-on mode and vice versa, a minimum power mode, a maximum driving force mode, proportioned modes wherein the armature position is dependent upon the current in the electromagnet coil or where force output is made proportional to the input current in the electromagnet coil, and in a latching mode.
Although such electromagnetic devices have been utilized, the diameter of the unit perpendicular to the armature axis must be large to minimize flux leakage. This large unit diameter makes it difficult to place several devices adjacent to each other within a limited space. Additionally, the elements of the device are not symmetrically placed. Accordingly, changes in temperature, which can effect magnetic force output, may produce changes in operation of the unit.
Another previously known electromagnetic reciprocating device has been designed to overcome many of these disadvantages by employing a single electromagnet with a long axial length with respect to its diametrical dimensions and a concentric elongated permanent magnet having radially oriented poles. The permanent magnet provides two oppositely directed flux paths flowing from the center of the armature towards its ends and a pair of pole pieces and back through an external shell to the permanent magnet.
At least one difficulty, however, with such a modified design is that radial magnets require special fabrication and tend to provide a relatively weak flux path across the armature gaps.
Still further previously known force motor units have required specialized assembly techniques and when the unit was employed in a liquid immersed environment electrical lead wires to the assembly had to pass through hydraulically sealed electrical connections.
The difficulties suggested in the proceeding are not intended to be exhaustive, but rather are among many which may tend to reduce the effectiveness of prior electromagnetic force motor devices. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that electromagnetic force motor devices appearing in the past will admit to worthwhile improvement.
In the above connection, it would be highly desirable to provide an electromechanical force motor which is symmetric while utilizing axially oriented permanent magnets. Additionally, it would be desirable to provide a force motor which eliminates costly and delicate centering springs while providing an accurate and low friction bearing arrangement for the armature. Further, it would be desirable to provide a force motor capable of operation in varying ambient environments without requiring costly sealing features in the force motor casing.